This invention relates to in situ single crystal growth on an inert substrate, and more particularly to growth of an array of semiconductor crystals.
There is considerable interest currently in in situ single crystal growth, i.e., growth at the site of ultimate use, most notably with Si, and in the integrated circuit context. One epitaxial growth system, silicon on sapphire (SOS), has competed directly with mainline Si technology, and other laser-utilizing growth schemes could supplement the mainline technology, as prototypes have demonstrated. Yet other possibilities for in situ single crystal growth exist, however, in perhaps less demanding contexts, e.g., in discrete devices like detectors or in photovoltaic devices.
Many schemes for in situ growth have the substrate play a direct role in nucleating crystal growth, e.g., in SOS and graphoepitaxy. Another approach, however, is to use the substrate in a more traditional role as a means to contain a liquid growth medium. In the context of the present invention, this approach could be called a "two-dimensional Bridgman."
Instances of the substrate mainly providing mechanical support for a thin film of liquid material were recently discussed by T. F. Kuech and J. O. McCaldin ("Confining substrate for micron-thick liquid films," Appl. Phys. Lett. 37, 44, July 1, 1980) who demonstrated how suitable contours in the surface of a substrate enable it to hold a thin liquid film captive on its surface. The liquid film is so confined due to an interplay between geometry and surface tensions. Although not much discussed in that paper, a striking feature of those experiments was the relative inertness of an SiO.sub.2 substrate toward nucleating crystal growth. Thus in "two-dimensional Bridgman" growth, one can have the opposite of the situation in graphoepitaxy where the substrate is active in nucleating crystallization.
The efficacy of variously shaped depressions in confining liquid stably, however, varies widely, as more recently discussed by the same authors ("Stability and pinning points in substrate confined liquids" J. Appl. Phys. 52, 803, February, 1981). The problem of obtaining liquid confinement in a substrate surface is twofold: first, the liquid must be put into some desired configuration and, secondly, this configuration must be stable, at least against small perturbations.
Most in situ crystal growth methods currently practiced of necessity employ lasers, electron beams, moving graphite heater strips, or similar dynamic heating means. Such dynamic heating means make control of the crystal growth process more difficult than is the case in conventional crystal growth processes, such as the Czochralski and Bridgman methods. To the contrary, the near-equilibrium growth methods proposed in this application utilize temperatures that are either uniform or very close thereto, so that our methods enjoy the same ease of control as the conventional crystal growth methods.
There are various technologies for the deposition of films and coatings. These are physical vapor deposition (PVD) consisting of evaporation, sputtering, ion plating; chemical vapor deposition (CVD) and plasma-assisted chemical vapor deposition (PACVD), electrodeposition and electroless plating; thermal spraying; plasma spraying; and detonation gun technologies.
The crystals of primary interest for growth by the present invention are the semiconductors. Applications where in situ crystal growth of these materials would be useful are manifold, most notably in arrays of devices like detectors, solar cells, and possibly integrated circuits. The characteristic dimension in most of these devices is a few microns, and an extensive lithographic art is now available to fashion structures to such dimensions. The present invention is primarily intended for structures of such size. However, the applicable techniques would permit a wide range of sizes which can equally well benefit from the present invention.
In undertaking the task of growing semiconductor thin-film crystals, it is necessary first to determine what substrate materials are most appropriate. Many materials have been considered in the solar cell art: Mo, W, Kovar (an iron-nickel-cobalt alloy), various glasses, to name a few. For the present invention, materials that interact little with the liquid are to be preferred. Amorphous SiO.sub.2 is such a material, though Si.sub.3 N.sub.4 may interact negligibly in some cases. Sintered graphite, ceramic and cermet are other examples. The implication for the present invention is that useful contact angles .theta. between liquid and substrate are likely to be large, most probably in the range 90.degree.&lt;.theta..ltorsim.150.degree., which is approaching what is often called "nonwetting". The problem is then to confine liquid material on the surface of the substrate while crystals are grown from the material using a selected one of the various technologies available.